How did an over-charging scandal involving 1,400 people at a single banking group, in the summer of 2015, grow to encompass 8,200 customers across 15 lenders by last Monday, and then climb to 15,000 cases, and counting, by yesterday?

The Central Bank has laid bare the extent of the tracker mortgage scandal, but only expects its investigation to lead to all 15 lenders under investigation to start compensating customers by the middle of next year.